1. Field of the Invention
This invention concerns a weakly acidic cation exchange resin having a moderate acidity and a process for producing the same. More specifically, this invention concerns a weakly acidic cation exchange resin having a moderate acidity, a small swelling volume and a great ion-exchange capacity, as well as a process for producing such cation exchange resin. This invention also relates to a porous weakly acidic cation exchange resin having a moderate acidity, a small swelling volume and a great ion-exchange capacity, as well as a process for producing such cation exchange resin.
2. Description of the Prior Art
Weakly acidic cation exchange resins generally employed at present are those of polyacrylic acid type or of polymethacrylic acid type. With the recent progress in the technics for using ion exchange resins, the acidity of these weakly acidic cation exchange resins has become an important factor.
For instance, in heat-regenerative ion exchange resins, it is required that the weakly acidic cation exchange resin and the weakly basic anion exchange resin as used have dissociation constants of good matching to each other. When a cross-linked polyvinylbenzyldimethylamine is used as the weakly basic anion exchange resin, a weakly acidic resin should have a greater acidity than that of the polyacrylic acid type ion exchange resin. A phosphorylated cation exchange resin has a greater acidity than the polyacrylic acid type resin, and it is put to practical use but is not suitable for the heat-regenerative ion exchange resin, since its acidity is too high. There is known no practical resin having a moderate acidity, that is, higher than that of a polyacrylic acid type resin but lower than that of a phosphorylated resin.
It is considered that a weakly acidic cation exchange resin having such a moderate acidity will provide a possibility for the applications to various fields where conventional polyacrylic acid type resins and polymethacrylic acid type resins could not be employed. Examples of the applications are for a buffer in low pH solution, for separation and purification of natural products, and for conversion of carboxylic acid salts having no electron attractive groups at .alpha., .beta. positions into free carboxylic acids as well as for to the heat-regenerative ion exchange resins.
As a weakly acidic cation exchange resin of carboxylic acid type other than the poly(meth)acrylic acid type, reaction products of polyvinyl alcohol in spherical particles and glyoxylic acid are known. For instance, an ionic exchange resin prepared by acetalizing a polyvinyl alcohol in spherical particles through reaction with glyoxylic acid in an aqueous solution containing sulfuric acid and sodium sulfate.
The above reaction is considered to proceed as follows: ##STR1##
The acetalization products prepared by this process are not, however, practical as an ion exchange resin since the swelling volumes of the products goes greater and accordingly the exchange capacity per unit volume is reduced as the exchange capacity increases. For instance, while the products obtained by the process have a swelling volume of 9.0 ml/g for the exchange capacity of 1.49 meq/g, the swelling volume increases to 13.6 ml/g for the exchange capacity of 2.42 meq/g and to 15.4 ml/g for the exchange capacity of 3.80 meq/g, respectively. Such a resin has an insufficient exchange capacity per unit volume and is liable to deform by pressure. So it cannot be considered as a practical ion exchange resin. Accordingly, the above process has a drawback in that it can only produce those weakly acidic cation exchangers having small exchange capacity.